Thin film transistors (TFTs) are known which utilize a thin film semiconductor formed on a substrate. The TFTs are utilized in integrated circuits, especially in an electro-optical device such as an active matrix type liquid crystal device as switching elements for each pixel or as driver elements in a peripheral circuit for driving active matrix elements.
Amorphous silicon films are readily available for TFTs. However, the electrical characteristics of amorphous silicon films are low. For this reason, it is desired to use semiconductor films having a crystallinity, that is, polycrystalline, microcrystalline silicon, monocrystalline semiconductor or the like.
As a method for forming silicon films having a crystallinity (crystalline silicon, hereinafter), it is known to deposit an amorphous silicon film first and then crystallize it by applying heat or light energy such as laser light.
However, in the case of using heat energy, it is necessary to heat a substrate to a temperature 600.degree. C. or higher for more than 10 hours. For example, a Corning 7059 glass which is generally used as a substrate for an active matrix type liquid crystal device has a glass distortion point of 593.degree. C. Accordingly, the crystallization through such a high temperature heat treatment is not desirable for a glass substrate. On the other hand, a short pulse laser such as an excimer laser has an advantage that it does not cause a distortion in a glass substrate. However, the uniformity of device characteristics is not so good in the case of using a laser. The inventors of the present invention considered that this is because of a temperature distribution in a laser beam.
The inventors of the present invention investigated a method for promoting a heat crystallization and a method for reducing a dispersion (ununiformity) in a laser crystallization in order to solve the problems concerning a crystallization of an amorphous silicon as discussed above.
With respect to the heat crystallization, it has been confirmed by the inventors that an amorphous silicon film can be crystallized through a heat treatment at 550.degree. C. for 4 hours by depositing a small amount of nickel, palladium, lead or the like on the silicon film.
As a method for introducing a small amount of the foregoing elements (i.e. a catalyst element for promoting crystallization), it is possible to use a plasma treatment, evaporation and ion implantation. The plasma treatment is a method in which a plasma of nitrogen or hydrogen is produced using an electrode including the catalyst element in a parallel plate type or positive columnar type plasma CVD apparatus, thereby, adding the catalyst element into an amorphous silicon film.
However, it is not desirable if the foregoing elements exist in a semiconductor too much because reliability or an electrical stability of a semiconductor device using such a semiconductor is hindered. Accordingly, the inventors have found that catalyst elements need to be used for crystallizing an amorphous silicon but it is desirable that a concentration of the catalyst elements in the crystallized silicon film be minimized. In order to achieve this object, it is desirable to use a catalyst element which is inactive in a crystalline silicon, and to accurately control the amount of the catalyst to be added into the silicon film in order to minimize the concentration of the catalyst element therein.
The crystallization process using a plasma treatment for adding nickel as a catalyst was studied in detail. The following findings were obtained as a result:
(1) In case of incorporating nickel by plasma treatment into an amorphous silicon film, nickel penetrates into the amorphous silicon film to a considerable depth before subjecting the film to a heat treatment; PA1 (2) An initial nucleation occurs from the surface of the film in which nickel is added; and PA1 (3) When a nickel layer is formed on the amorphous silicon film by vapor deposition, the crystallization of an amorphous silicon film occurs in the same manner as in the case of effecting the plasma treatment. PA1 forming an amorphous silicon film; PA1 introducing crystal nuclei to said amorphous silicon film; and PA1 growing crystals from said crystal nuclei, thereby obtaining a crystalline silicon film. PA1 (a) It is possible to accurately control and reduce the concentration of a catalyst element in the silicon film. PA1 (b) If the solution contacts a surface of an amorphous silicon film, the amount of the catalyst element to be incorporated into the silicon film is determined by the concentration of the catalyst element in the solution. PA1 (c) It is possible to introduce the catalyst element into the amorphous silicon film at a minimum density since the catalyst elements which are adsorbed by the surface of the amorphous silicon film function to promote the crystallization. PA1 (d) A crystalline silicon film having a good crystallinity can be obtained without a high temperature process.
In view of the foregoing, it is assumed that not all of the nickel introduced by the plasma treatment functions to promote the crystallization of silicon. That is, if a large amount of nickel is introduced, there exists an excess amount of the nickel which does not function for promoting the crystallization. For this reason, it is the point or the face of the silicon which contacts nickel that functions to promote the crystallization of the silicon at lower temperatures. Further, it is concluded that nickel has to be minutely dispersed in the silicon in the form of atoms. Namely, it is assumed that nickel needs to be dispersed in the vicinity of a surface of an amorphous silicon film in the form of atoms, and the concentration of the nickel should be as small as possible but within a range which is sufficiently high to promote the lower temperature crystallization.
A trace amount of a catalyst element capable of promoting the crystallization of silicon can be incorporated in the vicinity of a surface of an amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous concerning the controllability of the film, and is therefore not suitable for precisely controlling the amount of the catalyst element to be incorporated in the amorphous silicon film.
Next, with respect to a dispersion in a characteristics occurring in a laser crystallization, the inventors of the present invention found through experiments that this is caused by mainly the two reason, i.e. (1) a nonuniformity in a crystallinity due to a temperature distribution on a laser irradiated surface, and (2) the creation of crystal nuclei being contingent. Specifically, a laser beam generally has an intensity distribution in accordance with a gaussian distribution. The temperature of an amorphous silicon film is also in conformity with this distribution. As a result, during a crystallization of amorphous silicon through melting or partial melting, the crystallization must start at a region which has a lower temperature or a higher temperature dispersion than other regions because a crystallization occurs when a region from a melting condition to a solid phase. However, in practice, a crystal nuclei does not necessarily exist in such a region and therefore, there is a possibility that a supercooling region is formed. If such a supercooling region contacts crystal nuclei, a crystallization occurs explosively. Also, it is assumed that a uniform crystallization is difficult because the crystal nuclei tend to be formed at a surface roughness of an interface with a silicon oxide.
Accordingly, it is desired that a region at which temperature firstly becomes below a melting point among other regions is in conformity with a region in which crystal nuclei exist.